1. Field of the Invention
The present invention relates to bidirectional switches, currently called triacs, of medium power.
2. Discussion of the Related Art
Generally, a first surface, or rear surface of a triac is meant to be connected to a radiator (heat sink) and is covered with a uniform metallization forming a first main terminal of the triac. The opposite surface, or front surface, is covered with a second metallization of large dimensions forming the second main terminal of the triac, and one or more secondary metallizations, at least one of which forms a control terminal or gate of the triac.
Generally, the control terminal is referenced with respect to the second main terminal located on the same front surface. This can be a disadvantage, especially when it is desired to implement, monolithically or not, several triacs having a common main terminal (first or second) which is generally connected to ground that are also mounted on a radiator. In such an arrangement, the first main terminals (i.e., those on the rear surface) of the several triacs would be connected together and mounted on a common radiator connected to ground. The gates of the several triacs would then be controlled by a signal that is referenced to the second main terminals (i.e., those on the front surface) which are at high and possibly different potentials. To selectively control the different triacs, a control circuit needs to be provided, the reference voltages of which are high and possibly distinct voltages. Thus, the problem of implementing relatively complex control circuits arises. Alternatively, discrete triacs, having their second main terminals (those on the front surface) commonly connected to ground and having their rear surfaces mounted on a radiator for cooling and being at different potentials (which results in the necessity of providing one radiator per triac or isolated assemblies) have to be used.
This situation appears in a great number of devices. For example, in a washing machine, several triacs are used for controlling the pumps, the solenoid-operated filling valves, the various distributors, etc. Such medium power triacs are controlled by the same programmer unit and their main terminals on which the gates are referenced are interconnected. Since these main terminals are disposed on the same side as the gate, it is not possible to implement a monolithic structure and braze the metallization corresponding to the common terminal on a same radiator, because the gate metallizations disposed on the same surface would then be short-circuited. Such configurations are to be found in many other systems, for example, in controls of rolling shutters in which the engines include a coil for the closing of the shutter and another coil for its opening.